A modifier is any bonus or penalty applying to a die roll. A positive modifier is a bonus, and a negative modifier is a penalty. In most cases, modifiers to a given check or roll stack (combine for a cumulative effect) if they come from different sources and have different types (or no type at all), but do not stack if they have the same type or come from the same source (such as the same spell cast twice in succession). If the modifiers to a particular roll do not stack, only the best bonus and worst penalty applies.
A circumstance bonus (or penalty) arises from specific conditional factors impacting the success of the task at hand, such as the presence of precisely crafted tools in a workshop. Circumstance bonuses stack with all other bonuses, including other circumstance bonuses, unless they arise from essentially the same source. Circumstance modifiers are, of course, determined entirely by the GM.
A deflection bonus is an increase to Armor Class caused by physical or supernatural increases in a character’s ability to block and redirect incoming attacks. Deflection bonuses are provided by shields and by most track abilities that increase Armor Class.
A bonus granted because of a feat chosen by a particular creature. Feat bonuses do not stack with each other.
An item bonus or penalty is derived from an item (other than a shield) in a creature’s possession.Only one item bonus and one item penalty can ever be applied to a given d10 roll.
A bonus granted because of innate characteristics of that type of creature. If a creature’s race changes (for instance, if it dies and is reincarnated), it loses all racial bonuses it had in its previous form.
A size bonus or penalty is derived from a creature’s size category. Size modifiers of different kinds apply to Armor Value, attack rolls, and various other checks.
A bonus or penalty that is not given a specific type stacks with all other bonuses and penalties, including other untyped modifiers, as long as the various untyped modifiers don’t come from the same source (such as casting a spell on yourself twice).